The success of most drying processes is defined by the final moisture content of the product. Some dryer installations perform more than simply drying -- applications where the color, particle or material configuration, or integrity, size, etc., also influence the performance. But, for most, the final moisture is key. In any drying system, many variables can be altered to achieve a specification or control the operation. Typical variables to control dryer operation include energy (or air inlet temperature and mass) and feed rate. In some cases, the air volume or system pressure (differential or static) is varied to control the operation. System variables that require control include moisture level of the feed, feed temperature, instantaneous feed rate and, to some extent, ambient conditions.
All methods of controlling a dryer require the measurement of some variable that is indicative of the product's state. Possible variables include the exhaust gas and product temperatures, exhaust humidity, product final moisture content and product temperature. With the exception of the product's final moisture content, these parameters are all theoretically derived and the final control setpoint established during commissioning. The theory behind the derivation includes a heat and mass balance where all inlet and discharge data are defined. This information is obtained using clients' requirements, physics and empirical data. Calculations -- tempered by experience -- will define the initial value of the control parameter.
Most systems will have a single control loop, in that one variable will be the controlling parameter. Multiloop or cascade control, which employs more than one controlling parameter or controlling algorithms, provides more process benefits but also has more limitations. Clearly, different types of dryers are more or less suited to a particular control philosophy. The dynamics of the particles in each type of drying system vary, and as such, offer different techniques of being presented to a sensing instrument.
An obvious requirement to enable control using these techniques is that the parameter being measured must be presented to the instrument in a continuous, stable fashion. Suppose one elects to control on product temperature, for example. If the system experiences a blockage and the product is no longer presented to the instrument, system control is lost. Instability results.
An operational inefficiency associated with theoretically derived setpoints is the probability that more energy is being used than actually is required. This is due purely to the contingency in the calculations. These safety factors could potentially overdry the product, which may be detrimental if specific final product moisture values are required. Fortunately, in most applications, the control setpoint value is optimized during commissioning, eliminating this inefficiency to some extent.
One drawback is that these control methods are reactionary - the point of measurement is after the operation. Sometimes, the system's response time may be too slow and off-spec product will be produced. But, if one places the instrument too close to the feed, the operation beyond the control point will be unmonitored. Feed temperature control is essential for temperature-sensitive products.
Two primary variables are modulated to achieve control: inlet temperature and feed rate.
Modulating the Inlet Temperature. This method is robust and forgiving of system feed fluctuations. The feed rate remains constant (after an initial ramp-up period) and the inlet temperature modulates to achieve the control setpoint. Should the inlet moisture content vary, the system will respond by applying more or less energy to the operation as required. The same holds true for instantaneous variations in the feed rate. This control should operate within limits to which the system can react within the control time cycle.
Modulating the Feed Rate. The other common method of controlling the drying process is to modulate the feed rate based on fluctuations in the control parameter. With this approach, inlet temperature is maintained at a constant value. This method is most effective if the feed is sensitive to overheating, if product reaction or damage can occur at high temperatures, or if the heat source must be operated at its full output to be most effective, efficient and stable.
The mechanical feed method requires a variable speed control that will increase or decrease the feed rate to the dryer. The system's response time must be relatively fast or the final product will exhibit corresponding variations in moisture content.
One weakness of this type of control occurs during startup and shutdown. If there is no load on the system, the exhaust temperature will converge to that of the inlet temperature (once the system reaches its soak temperature). This could cause significant damage to the fabric of postdrying operations should a feed stoppage occur during steady production.
In Part 2, I'll look at controlling on parameters such as exhaust temperature, humidity or final moisture content.
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